File restructuring

In response to Issue #24

This branch reorganizes the codebase of separate APIs into individual files. Every API that is a not part of the core implementation of a service is now named service_api.go. This restructuring is meant to accomplish several things:

1. Closer mapping to the structure to the GitHub API. For example, http://developer.github.com/v3/repos/ -> repos.go, http://developer.github.com/v3/repos/hooks/ -> repos_hooks.go.
2. As a result, better organization and therefore navigation of files. Instead of looking through repos.go for the Hooks API implementation, you can now just go to repos_hooks.go.
3. Clear structure of files while keeping the code under one github package. Adding folders = adding packages. This makes it impossible to use a single github package due to dependencies, and would require us to change how clients fundamentally make API calls.
4. Provide better organization while minimally affecting clients. I did merge the events API into its proper place within the activity service. Since this removes EventService, clients will have to s/EventsService/ActivityService/g their code. If we really need to, we can put back the EventsService for legacy reasons. Otherwise, this is mostly an organizational change and does not change usage.